Measuring Air Flow ..2

This is page two of our discussion about SCFM and how it affects compressed air measure and flow. Page one on SCFM is right here.

As a home user of an air compressor and air tools, or if you are an industrial user of compressed air, why do you want to know the definition of SCFM?

Ignore SCFM?

Really, you do not.

You do need to know how much flow and pressure a specific compressor will generate for you.

You will know how much air and at what air pressure a specific air tool or compressed air using application will need to ensure that your applications have enough compressed air flow and pressure to do their work! In most cases, that is all you care about.


CFM & Pressure

When you first start to source an air compressor then you will first need to know how much compressed air your equipment will use in CFM and at what air pressure that CFM must be.

Your air-driven equipment will often have a tag that identifies the flow requirements, or, you can check the manufacturers web site for that information, or to obtain a contact from whom you can obtain that data.

Remembering that in compressors over 10 HP in size, each HP of motor capacity will generate about 4 CFM of compressed air at 90 PSI, and under 10 HP you should get 3-4 CFM of compressed air at 90 PSI for each HP of electric motor size.

Knowing this, and having researched to find out the number of CFM and pressure required to run your air equipment is, you can now source a compressor of the correct size.

When you find a compressor you think suits your needs, if it were me, I would provide them with your requirements in CFM at the pressure you need. Let the compressor manufacturer guarantee to you that their recommended air compressor will provide sufficient flow and pressure, regardless of what term they use to describe the compressed air discharge from their compressor.

Higher Discharge At Lower Pressures

You may need to be a bit careful about compressor flow claims. An air compressor will show a higher discharge flow rate at a lower pressure level than it will at a higher pressure level. Compressor manufacturers will often highlight a high flow rate and lowlight the CFM. It will likely be air flow at the higher pressure that you will require for your application!s

Make sure, when you are discussing your compressor size with your vendor, whether they measure the outflow flow from their compressor in SCFM or CFM or ACFM or whatever, insist that they assure you that you will get the CFM you need at the pressure you need for the compressed air equipment that you need to run.

I recently had a person send in some more information, a consideration of SCFM from an engineering perspective. I did not write this, though I am appreciative of the advice and happy to add it here for you.

An Engineering Perspective On SCFM

"Using scfm provides a more precise measurement of air flow.

Air gets denser as you compress it, less dense as you heat it. In order to provide accurate flow measurement to an end user or to provide an "apples to apples" comparison of flow, that variability has to be removed.

So a "standard" cubic foot was created - based as you've said - rather loosely on a standard pressure and temperature.

Typically being 1 atmosphere and temperature of either 60F, 68F or 15C. In other words, 1 scf is the space that 1 cubic foot of air occupies at atmospheric pressure and a standard temperature. At 90 psig that same cubic foot takes up a lot less space than it did at atmospheric pressure so 1 cfm @ 90 psi contains 7 scf.(standard cubic feet)

In real life, cfm is the useful output rate and it is a measure of exactly how many cubic feet leave the compressor at the rated pressure in a minute. There is no regard for the density of that cubic foot or whether or not my compressors cubic foot contains as much air as your compressor. That's where scfm comes in - it equalizes the playing field, providing standardized correction to capacity claims at 110 psig versus those at 90 or 150 psig for instance.

In reality no one using compressed air to drive a tool really cares about scfm - we're all concerned about how many cfm at my required pressure can i get so i know i can drive this air wrench etc. As long as i get 30 cfm of 90 psi air and my wrench works i don't care how many scf that air contains.

However if you deal with the accounting side of a large firm you'll know they bill or pay based on scfm used so that every cubic foot used is corrected for pressure and temperature, preventing over or under charging. It provides consistency day to day and season to season - same goes for the gas company, your bill will show use in scf (perhaps 1000's of scf)."

My thanks go to this contributor for their contribution.